A black light will explode (permanently destroying the light) when you are adjacent to it, causing you to hallucinate, but causing no other damage. The hallucination will last for 10d12 turns. Pets are also affected, so it is well worth protecting especially strong pets from hallicinogenic phenomena. Monsters don't have a confusion timer. Instead, they have a 2% chance to recover on each of their moves.[1]

As black lights can't see invisible, if you are invisible or displaced, they may explode at an empty square with no hallucination effect to the player, but potentially to a pet in the square targeted.